Aliphatic polyesters such as polyglycolic acid or polylactic acid are hydrolyzed in vivo and, in natural environments, are metabolized and degraded to water and carbon dioxide by microorganisms. Therefore, aliphatic polyesters have attracted attention as biodegradable polymeric materials which can be substituted for medical materials or commodity resins. Of these aliphatic polyester resins, polyglycolic acid has not only high biodegradability and hydrolyzability when an alkaline solution or the like, for example, is used, but also excellent mechanical characteristics such as heat resistance and tensile strength and, in particular, excellent gas barrier properties when used as a film or a sheet. Therefore, polyglycolic acid is expected to be used as agricultural materials, various packaging (container) materials, or polymeric materials for medical use, and applications have been expanded by using polyglycolic acid alone or combining polyglycolic acid with other resin materials or the like.
PGA can be synthesized by dehydrative polycondensation of glycolic acid serving as a monomer. However, with a polycondensation method using glycolic acid as a starting raw material, it is difficult to obtain high molecular weight PGA for use in a molding material or the like. Therefore, high molecular weight PGA is synthesized by performing ring-opening polymerization on a glycolide having the structure of a bimolecular cyclic ester of glycolic acid (may be called a “dimeric cyclic ester” hereafter) (that is, 1,4-dioxane-2,5-dione).
Specifically, PGA can be synthesized by dehydrating polycondensation of glycolic acid (that is, -hydroxyacetic acid) in accordance with the following formula [1]:

However, with a polycondensation method using glycolic acid as a starting raw material, it is difficult to obtain high molecular weight PGA. Therefore, high molecular weight PGA (that is, polyglycolide) is synthesized by performing ring-opening polymerization on glycolide having the structure of a bimolecular cyclic ester of glycolic acid in accordance with the following formula [II] in the presence of a catalyst such as tin octanoate.

In order to mass-produce high molecular weight PGA on an industrial scale using glycolide as a raw material, it is indispensable to efficiently and economically supply high-purity glycolide. However, it was difficult to synthesize glycolide efficiently and economically. Glycolide is a dimeric cyclic ester with a structure in which two molecules of water are eliminated by an esterification reaction of two molecules of glycolic acid, but when glycolic acids are simply esterified with one another, a low molecular weight polymer such as a glycolic acid oligomer is ordinarily formed, and it is not possible to obtain glycolide as a dimeric cyclic ester with high yield. Therefore, a method of producing glycolide by synthesizing a glycolic acid oligomer and then depolymerizing the oligomer, for example, has been used.
The following is an example of a method conventionally known technique for obtaining a dimeric cyclic ester of -hydroxycarboxylic acid such as glycolide.
U.S. Pat. No. 2,668,162 (Patent Document 1) discloses a method of pulverizing a glycolic acid oligomer into a powder form, depolymerizing the ground product by heating to 270 to 285° C. in an ultra-vacuum of from 12 to 15 torr (1.6 to 2.0 kPa) while supplying the powder to a reaction vessel at a ratio of very small increments of approximately 20 g/hour, and then cooling, solidifying, and recovering the gaseous glycolide that is produced. In addition, Japanese Unexamined Patent Application Publication No. S63-152375A (Patent Document 2) discloses a method of using a polyether with excellent thermal stability as a substrate, performing block copolymerization on the substrate with a small amount of glycolic acid to form a block copolymer, and then depolymerizing the copolymer by heating so as to cool, solidify, and recover the gaseous glycolide. Further, U.S. Pat. No. 4,835,293 (Patent Document 3) discloses a method of heating a glycolic acid oligomer to form a melt, blowing an inert gas such as nitrogen gas onto the surface of the melt, making the glycolide that is produced and volatilized from the surface accompany the gas flow, and then cooling the gas flow to solidify and recover glycolide.
The glycolide obtained in this way contains impurities (primarily the glycolic acid oligomer that is used and the glycolic acid serving as a raw material thereof), so purification is conventionally performed by recrystallization using various different solvents such as, for example, isopropanol, t-amyl alcohol, carbon tetrachloride, and ethyl acetate. The slurry of crystalline glycolide obtained by recrystallization is washed with the solvent used in recrystallization or another washing solution while solid-liquid separation is performed by filtration, for example, and a purified crystal is obtained by then removing the solvent or the washing solution by drying.
However, such a purification method needs to include a drying step for removing the solvent or the washing solution from the crystal surface, a cooling and recovery step for the solvent or the washing solution removed in the drying step, and a distilled separation step for a mixture of the recovered solvent and washing solution, and these steps are intricate. Drying is performed at a temperature equal to or less than the melting point of the crystal, but since cyclic esters such as glycolide are sublimable, the crystal loss becomes large when the degree of depressurization is made too high at the time of drying. Further, impurities may also be incorporated into the crystal, and removing the impurities requires several cycles of recrystallization, which makes the step even more intricate.
On the other hand, there is also a known method of producing a cyclic ester such as glycolide using a high-boiling-point organic solvent. Japanese Unexamined Patent Application Publication No. H9-328481A (Patent Document 4) discloses a method of using a high-boiling-point organic solvent in a method for producing a dimeric cyclic ester of -hydroxycarboxylic acid by depolymerizing an -hydroxycarboxylic acid oligomer. This production method is a method of heating a mixture containing from 30 to 5,000 parts by weight of a high-boiling-point organic solvent per 100 parts by weight of an -hydroxycarboxylic acid oligomer to a temperature at which depolymerization occurs so as to form an essentially uniform solution phase, further continuing heating at the same temperature to distill out the dimeric cyclic ester that is produced together with the high-boiling-point organic solvent, and then recovering the dimeric cyclic ester from the distillate. With this method, it is possible to obtain a dimeric cyclic ester from an -hydroxycarboxylic acid oligomer with high yield while preventing the oligomer from becoming a tarry material. In addition, Patent Document 4 describes a method of purifying a crude dimeric cyclic ester of an -hydroxycarboxylic acid by applying the method described above.
Further, WO2002/14303 (Patent Document 5) discloses a production method for a cyclic ester, wherein:
(I) a mixture containing an aliphatic polyester (A) and a polyalkylene glycol ether (B), which is expressed by the following formula:X′—O—(—R′—O—)a—Y′(wherein R′ is a methylene group or a straight-chain or branched-chain alkylene group having from 2 to 8 carbon atoms, X′ is a hydrocarbon group, Y′ is an aryl group or alkyl group having from 2 to 20 carbon atoms, a is an integer of 1 or greater, and when a is 2 or greater, a plurality of R′ moieties may be the same or different from one another) and has a boiling point of from 230 to 450° C. and a molecular weight of from 150 to 450, is heated to a temperature at which the depolymerization of the aliphatic polyester (A) occurs at normal pressure or reduced pressure;(II) a substantially uniform solution phase is formed in which a melt phase of the aliphatic polyester (A) and a liquid phase consisting of the polyalkylene glycol ether (B);(III) heating is continued in the solution state so as to produce the cyclic ester by depolymerization and distil out the cyclic ester together with the polyalkylene glycol ether (B); and(IV) the cyclic ester is recovered from the distillate. According to this method, the cyclic ester produced by depolymerization is distilled off together with the polyalkylene glycol ether and both compounds are separated into distinct liquid phases to recover the cyclic ester phase, while the polyalkylene glycol ether phase without thermal deterioration may be circulated to the reaction system of depolymerization for its reuse. In addition, Patent Document 5 also describes a method of purifying a crude cyclic ester by applying the method described above.
Further, French Unexamined Patent Application Publication No. 2692263A (Patent Document 6) discloses a method of adding an oligomer of an -hydroxycarboxylic acid or an ester or salt thereof to a solvent containing a catalyst and stirring while heating to achieve catalytic decomposition. This method is performed at normal pressure or increased pressure using a solvent suitable for accompanying a cyclic ester in the gas phase, and the gas is condensed to recover the cyclic ester and the solvent. The Patent Document 6 illustrates a specific example in which a lactic acid oligomer is subjected to catalytic cracking using dodecane (boiling point: approximately 214° C.) as a solvent. However, when the present inventors conducted additional tests under the same conditions using a glycolic acid oligomer and dodecane, it was demonstrated that the formation of tarry material progresses simultaneously with the initiation of the depolymerization reaction and that the production of glycolide stops at a point when only a very small amount of glycolide has been produced. Moreover, the reaction residue was viscous, and cleaning required a substantial amount of labor. It is presumed that glycolide is susceptible to hardening by ring-opening polymerization within the device since it has higher reactivity than lactides.
With these methods, a dimeric cyclic ester of an -hydroxycarboxylic acid such as glycolide is distilled out together with a high-boiling-point organic solvent. The recovery of the cyclic ester such as glycolide from the distillate is performed by cooling the distillate, further adding a non-solvent of the cyclic ester such as glycolide as necessary, solidifying and precipitating the cyclic ester such as glycolide, and then performing solid-liquid phase separation. However, a crystal of a cyclic ester such as glycolide obtained in this way has low purity and not only contains impurities, but a high-boiling-point organic solvent that is difficult to remove by ordinary drying is deposited on the crystal, as described above. Therefore, in order to obtain a dry crystal with high purity, an operation of removing the organic solvent deposited on the crystal is essential in addition to the purification operation described above.
A conventional method of purifying a cyclic ester such as glycolide and removing an organic solvent deposited on the crystal is performed by substituting and washing the resulting crystal with a low-boiling-point washing solution such as cyclohexane or an ether and then removing the washing solution by drying. Drying is performed at a temperature equal to or less than the melting point of the crystal, but since cyclic esters such as glycolide are sublimable, the crystal loss becomes large when the degree of depressurization is made too high at the time of drying, and the yield of the cyclic ester such as glycolide decreases. Recrystallization is also sometimes further performed with ethyl acetate or the like as necessary, and the organic solvent deposited on the crystal is removed by drying at this time as well. Therefore, in a conventional method of purifying a crystal of a cyclic ester such as glycolide on which a high-boiling-point organic solvent is deposited, a new low-boiling-point washing solution becomes essential for substituting the organic solvent deposited on the crystal. As a result, the washing waste solution becomes a mixture of the high-boiling-point organic solvent and the low-boiling-point washing solution.
The aforementioned purification method for a crystal of a cyclic ester such as glycolide includes a drying step for removing the washing solution from the crystal surface, a recovery step for the washing solution removed by drying, and a purification and recovery step for the washing waste solution containing the high-boiling-point organic solvent and the low-boiling-point washing solution, and the steps are intricate. In addition, the washing solution that is used, such as alcohol, for example, may react with the cyclic ester and cause a transesterification reaction. Further, when impurities are incorporated into the crystal, several cycles of recrystallization are necessary, which makes the step even more intricate.
Therefore, in a method of producing glycolide by means of the depolymerization of a glycolic acid oligomer, there has been a demand for a method of producing a high-purity glycolide efficiently and economically, whereby complex purification operations can be reduced or, preferably made unnecessary.
WO2006/129736 (Patent Document 7) describes a method for producing a dimeric cyclic ester comprising two steps: [first step] a step in which a polymerization solution is obtained by adding alkylene glycol having a higher boiling point than the dimeric cyclic ester to be produced to at least one of an -hydroxycarboxylic acid such as glycolic acid and an -hydroxycarboxylic acid condensate such as glycolic acid condensate and performing a polymerization reaction; and [second step] a step in which the polymerization solution obtained in the first step is heated at normal pressure or reduced pressure, and a dimeric cyclic ester is obtained by performing a reaction and distillation simultaneously. As a specific example, a glycolic acid condensate is placed in a 500 ml flask, and polyethylene glycol is added (liquid, boiling point: 314° C., weight average molecular weight: approximately 400). The mixture of the glycolic acid condensate and the polyethylene glycol is then heated to 230° C. in a nitrogen atmosphere under reduced pressure conditions of 1.0 kPa so as to promote a polymerization reaction. A depolymerization reaction begins when heating is continued further, and the glycolide, which is a dimeric cyclic ester, is distilled out and accumulated in a receptacle. The second step then begins, wherein the mixture is heated at the temperature described above until the distillation of the glycolide substantially stops, and the glycolide is collected. It is described that when the content of the flask was observed after distillation was complete, a residue was present in the flask, and the deposition of the distillate was observed in the distillation line between the flask and the receptacle, but the amount of accumulation was minimal.
Polyglycolic acid is expected to be mass-produced and used in large quantities in the future, and the recycling of the product waste will be a critical issue. The recycling of molding wastes produced as a by-product at the time of the molding of polyglycolic acid will also become an issue. In a method of producing glycolide by means of the depolymerization of a glycolic acid oligomer, there is a demand for a method of stably producing high-purity glycolide efficiently and economically.